168 LESLIE B. AREY AND W. J. CROZIER 



small (2 mm. long), and when taken from a chiton at low tide 

 their coloration is quite pale, of a yellowish cast, with minute 

 black markings. The coloration becomes darker in the light, and 

 then reproduces on a small scale something of the greenish-to- 

 black color pattern of the chiton girdle. The sphaeromas fre- 

 quently remain in place under the chiton at high tide, and in a 

 glass aquarium they will reassume a position within the gill 

 channel or under the girdle of medium-sized or large chitons. 

 There they take up stations chiefly along the lateral margins 

 of the girdle, which are shghtly raised during the respiration of 

 the chiton. The isopods remain with heads pointed outward, 

 into the incoming respiratory current. Sudden shading causes 

 them to dart back into the ctenidial channel. When in their 

 resting position a small portion of the anterior end may project 

 beyond the edge of the girdle of their host, and under these cir- 

 cumstances their coloration, resembling that of the dorsal tu- 

 bercles upon the girdle, renders them very inconspicuous. Occa- 

 sionally one of these isopods creeps in between the gill filaments, 

 usually at the posterior end of a chiton, and under these condi- 

 tions is forcibly shot out at the posterior extremity of its host by 

 means of the water current; the isopods appear, moreover, to 

 react negatively to currents of this strength, and continue vio- 

 lently to swim away, in a spiral path, from the region of the anal 

 current even after it has ceased to act upon them in a gross 

 mechanical way. At high tide, and when under water in aquaria, 

 the isopods creep freely over the dorsal surface of the chitons; 

 under these circumstances their coloration is to a very high 

 degree homochromic and concealing. This relation between 

 Sphaeroma and Chiton will be made the subject of further study. 

 At present it can be said that there does not appear to be any 

 precise 'attraction' (chemical, for example) exerted by the chitons 

 upon the isopods. 



This commensal isopod is involved in the very complex envir- 

 onmental correlations, which may be clearly analyzed, in the 

 life-history of Chitons. Therefore we mention it here, although 

 detailed work on its behavior and relations must be deferred for 

 the present. 



